Eat Now

The daily wear and tear experienced by our cells can damage internal organelles, such as the mitochondria. The recognition and subsequent removal of injured mitochondria rely on a sequestration and engulfment process known as mitophagy, but the molecular pathways involved are unclear. Mitophagy plays a critical role in cellular health and in quality control, and its disruption has been implicated in neurodegeneration, diabetes, cardiac disease, and cancer. Working with cultured neuronal cells, Chu et al. now describe an “eat me” signal that identifies injured mitochondria as being fit for consumption by the intracellular recycling machinery. Cardiolipin is a mitochondria-specific phospholipid that normally resides within the inner mitochondrial membrane. The collapse of membrane asymmetry in mitochondria owing to damage inflicted by a variety of pro-mitophagy agents leads to the exposure of cardiolipin on the outer mitochondrial membrane, which marks them for delivery to lysosomes. The authors show that cardiolipin interacts with LC3, a protein known to be involved in autophagy; in particular, two arginine residues in the N-terminal region of LC3 stabilize the phosphate groups of cardiolipin, and subtracting the guanidinium side chains via mutation abrogated the binding of LC3. Thus, cells protect themselves from the potentially deleterious effects of damaged mitochondria by exploiting the damage-induced redistribution of cardiolipin.

Nat. Cell Biol.15, 1197 (2013).

Neuroscience

Sleep Now

Lisa D. Chong

Neuropeptide Y (NPY) acts in the mammalian brain to control numerous behaviors, including food intake and sleep. In Drosophila, there are two NPY-like peptides, neuropeptide F (NPF) and also a short version (sNPF). Shang et al. have found that the activation of sNPF-expressing neurons in the fly brain promotes sleep. When the activity of these neurons was enhanced, flies were prone to sleep more; later, when these neurons were silenced, flies actually slept less, as if their internal clocks had registered the additional time already spent sleeping. Conversely, experimentally activating these neurons during a period of sleep deprivation (achieved by incessant agitation on a vortex mixer) reduced the amount of catch-up sleep needed. These neurons are controlled by wake-promoting neurons that express the neurotransmitter γ-aminobutyric acid and suppress the activity of the sNPF neurons, reducing daytime sleep. Finally, the effects of sNPF on feeding appear to be secondary to its effects on sleep.

Neuron80, 171 (2013).

Climate Science

Historical Carbon Uptake

The high levels of atmospheric CO2 that have resulted from fossil fuel burning and other anthropogenic activities over the past 150 years are expected to cause increased uptake of carbon by the terrestrial biosphere over the next century, thereby partially offsetting some of the CO2 emissions. This effect is difficult to quantify, though. Deforestation and other land-use changes have transferred great quantities of carbon from the biosphere to the atmosphere since preindustrial times, making the magnitude of carbon uptake by land plants difficult to infer. Shevliakova et al. address this issue with a coupled climate–carbon cycle model study of the terrestrial carbon sink, paying special attention to how land use has changed since the beginning of the industrial revolution. They estimate that enhanced vegetation growth over that period reduced atmospheric CO2 concentration by 85 ppm below what it would have been without that effect, thereby avoiding approximately 0.3°C of warming. This represents a dramatic shift of the carbon budget, by more than 250 billion tons of carbon—more than 30 years of emissions at current rates.

Proc. Natl. Acad. Sci. U.S.A.110, 16730 (2013).

Materials Science

Tracking Graphene Growth

Phil Szuromi

A convenient route for preparing monolayer graphene is via chemical vapor deposition of hydrocarbons onto copper substrates held at elevated temperatures. Kidambi et al. examined how changes in process conditions affect the growth chemistry with several in situ techniques, including x-ray photoelectron spectroscopy (XPS) and environmental scanning electron microscopy (ESEM). They used benzene as the hydrocarbon source (at partial pressures up to ∼4 torr) at substrate temperatures of 900°C. During growth in the absence of air, a shift in the main C1s XPS peak indicated that graphene was coupled to metallic copper. After cooling in vacuum and air exposure, the main carbon feature underwent a shift indicative of decoupling of the graphene from the surface via oxygen intercalation, rather than surface oxidation. Visualization of this decoupling with ESEM revealed an island-growth process proceeding from the edges of the island inward. However, if air was present during growth, there was an increase in the carbon peak associated with carbon deposition at defect sites; surface oxide formed, and roughly half of the graphene present was decoupled from the surface.